There are means for allowing large-capacity communication without expanding the frequency band, including MIMO (Multiple-Input Multiple-Output) techniques using a number of transmitting/receiving antennas (for example, see Patent Documents 1 and 2). With space division multiplexing (SDM) is, which is a MIMO technique, a number of antennas are provided in the transmitter and receiver, and the transmits independent signals different from one another from the antennas and spatially divides and the receiver receives a number of different signals simultaneously transmitted by directivity of a number of antenna array in receivers, thereby enabling communication capacity to be increased.
FIG. 1 is a diagram showing an example of SDM communication. As shown in FIG. 1, the transmitter transmits multiplexed data streams from a number of antennas (substreams), and the receiver receives radio signals transmitted by a number of paths (crosstalk) and divides the multiplexed signals.
In addition, if propagation channel information is known at the transmitter end, directivity control (transmission beam formation) is performed using a specific vector obtained by performing single value decomposition on a matrix which makes each channel response of propagation channels an element and forming a spatial orthogonal channel, thereby enabling much larger communication capacity than SDM to be obtained.
FIG. 2 is a diagram showing an example of SDM communication that performs directivity control at the transmitter end. As shown in FIG. 2, the transmitter performs processing of transmission power control and beam forming on signals transmitted from the antennas, and forms channels that are spatially orthogonal.
In SDM communication that performs directivity control at the transmitter end, methods for knowing channel information on the transmitting end may include providing feedback of a channel estimation result at a receiving end, or performing channel estimation at a transmitting end.
However, in the actual propagation environment, channels change at all times. FIG. 3 is a diagram showing change in propagation environment. As shown in FIG. 3, channel environment A at a given time changes to channel environment A′ as time goes by.
When directivity control is performed using a result of channel estimation performed in environment A at the receiving end, if channel fluctuation is large—for example, if environment A differs from environment A′ largely—it is not possible to form spatial orthogonal channel, and error rate characteristic when directivity control is performed at the transmitting end may be deteriorated than SDM when directivity control is not performed at the transmitting end.
Similarly, when channel estimation and directivity control are performed at the transmitting end, a complicated calculation on the transmitting end such as channel estimation or singular value decomposition may cause processing delay, which result in deterioration of error rate characteristic when directivity control is performed than in normal SDM.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-258770    Patent Document 2: Japanese Patent Application Laid-Open No. 2001-505723